Typically, in laparoscopic, endoscopic, or other minimally invasive surgical procedures, a small incision or puncture is made in a patient's body. A cannula is then inserted into a body cavity through the incision, which provides a passageway for inserting various surgical devices such as scissors, dissectors, retractors or similar instruments. To facilitate operability through the cannula, instruments adapted for laparoscopic surgery typically embody a relatively narrow shaft supporting an end-effector (EE) at its distal end and a lever or handle at its proximal end. Arranging the shaft of such an instrument through the cannula allows a surgeon to manipulate the proximal handle from outside the body to cause the distal end-effector to carry out a surgical procedure at a remote internal surgical site. In most embodiments, the handle and tool shaft can be directly connected and roll rotation of the entire handle may drive rotation of the entire tool shaft and end-effector. Some alternative laparoscopic tools, such as, for example, U.S. Pat. No. 8,668,702 includes a handle that is not directly connected to the tool shaft but connected via an input joint (e.g., comprising a pair of transmission strips) which still allows for roll rotation of the tool shaft and end-effector by way of handle rotation. In general, a handle body may be referred to as a handle reference, or as a palm grip, handle shell, or the like.
A laparoscopic or endoscopic instrument may provide a surgeon with the ability to transfer high force loads from the proximal end of the tool to the distal end. These forces are transferred through the instrument through an input, output and transmission member sub-system, where the sub-system consists of a mechanism, as seen in most surgical instruments, such as U.S. Pat. No. 5,330,502. The input mechanism generally consists of an actuating lever body as an input and an output (for example, a shuttle coupled to the handle body via a one Degree of Freedom (DoF) slider joint). As a user actuates the handle lever, the motion is transferred to the shuttle, and the amount that the shuttle displaces is based on the input mechanism's mechanical advantage, or transmission ratio. The terms transmission ratio and mechanical advantage are both used in this document since the transmission ratio and mechanical advantage are, in general, simply the inverse of each other. When emphasizing force, the attribute mechanical advantage is used, and when emphasizing displacement, the attribute transmission ratio is used. Similarly, the output mechanism can have a varying mechanical advantage, or transmission ratio, over the output stroke. For a mechanical surgical instrument which requires a high force output while not compromising on output displacement, this varying mechanical advantage of both the input mechanism and the output mechanism will have a certain desirable profile. The mechanical advantage at the initial segment of the stroke can be low because no force build up is required initially; however the mechanical advantage at the end of the stroke needs to be high to allow a reasonable force input to be amplified into a large force output. The transmission members used in the prior art are generally stiff in the direction of transmission. However, this transmission member does not have to be rigid. In the transmission systems described herein, the transmission member itself is designed to have a finite stiffness so that it acts as an energy storage member during certain portions of the input stroke of the device. This offers a unique performance of the device and has many benefits over rigid or highly stiff transmission members.